Essential Linux Command Cheat Sheet: 20 Must‑Know Answers for Sysadmins

Q1: How to view the current Linux server runlevel? Use who -r or runlevel commands.

Q2: How to view Linux's default gateway? Run route -n or netstat -nr to display the default gateway and the routing table.

Q3: How to rebuild the initramfs image on Linux? On CentOS/RHEL 5.x use mkinitrd -f -v /boot/initrd-$(uname -r).img $(uname -r). On CentOS/RHEL 6.x use dracut -f for the current kernel or

dracut -f initramfs-2.x.xx-xx.el6.x86_64.img 2.x.xx-xx.el6.x86_64

for a specific kernel version.

Q4: What does the cpio command do? It copies files into or out of an archive, allowing you to create or extract archive files.

Q5: What is the patch command and how to use it? patch applies changes (diffs) to files. Create a diff with diff -Naur old_file new_file > diff_file, then apply it using patch < diff_file.

Q6: What is aspell used for? It is an interactive spell‑checker for Linux, a free alternative to ispell, usable both as a standalone tool and as a library for other programs.

Q7: How to view a domain's SPF record from the command line? Use dig -t TXT example.com (replace with the target domain).

Q8: How to identify the package that provides a specific file (e.g., /etc/fstab )? Run rpm -qf /etc/fstab.

Q9: Which command shows the status of bond0 ? Use cat /proc/net/bonding/bond0.

Q10: What is the purpose of the /proc filesystem? It is a memory‑based virtual filesystem exposing kernel and process information such as CPU, memory, partitions, I/O addresses, DMA channels, and running processes.

Q11: How to find files larger than 10 MB under /usr ? Execute find /usr -size +10M.

Q12: How to locate files in /home not modified in the last 120 days? Use find /home -mtime +120.

Q13: How to find files in /var not accessed in the past 90 days? Run find /var ! -atime -90.

Q14: How to delete all files named core in the entire directory tree without prompts? Use find / -name core -exec rm {} ;.

Q15: What does the strings command do? It extracts printable strings from binary files, useful for analyzing unknown binaries or investigating potential intrusions.

Q16: What is the purpose of the tee filter? It duplicates standard input to both a file and standard output (or another program). Example: ll /etc | nl | tee /tmp/ll.out.

Q17: What does export PS1="$LOGNAME@hostname:$PWD:" change? It modifies the shell prompt to display the username, hostname, and current working directory.

Q18: What does ll | awk '{print $3,"owns",$9}' do? It prints each file's owner (field 3) followed by the word "owns" and the file name (field 9).

Q19: What is the at command used for? It schedules a one‑time task to run at a specified future time, stored under /var/spool/at and executed by the atd daemon.

Q20: What does the lspci command display? It lists PCI bus devices and attached hardware; options -v, -vv, or -vvv increase verbosity, and -r improves readability.